As is well known, leakage flow from the pressure side to the suction side of a turbine blade across the tip (the gap between the blade tip and the outer air seal) results in a performance loss of aircraft gas turbine engines. There has been over the years a continual effort to maintain a close clearance between the tips of an axial flow turbine blades and the outer air seal surrounding these tips for the entire operating envelope of the engine. As one skilled in the art appreciates, because the rotor has a greater mass than the engine casing, the rotor will expand and contract in response to temperature changes slower than the casing. The engine is initially designed so that the tips of the blades will not rub against the outer air seal for both transient and steady-state conditions. Hence, the gap must be sufficiently large to accommodate certain transient conditions and yet be small when the engine is operating at a steady-state condition. This presents problems since the gap is designed to obviate rubbing to accommodate the transient conditions. When the engine returns to the steady-state condition the gap is generally larger than desired unless means are taken to adjust for this problem. This problem is acerbated when an engine, particularly powering military fighter aircraft, is put through extreme transient conditions such as throttle chops, rapid re-accels and the like which require the engine case and rotor components to respond more rapidly than would otherwise be the case in a commercial airline. Presently, there are two fundamental ways in which this gap is controlled, one by an active control system and the other by a passive control system. Essentially, an active control system, sometimes referred to as active clearance control, typically relies on some external heat or cooling source and the actuation of an external control system that serves to conduct the heating or cooling from the source to the component parts in proximity to the blade so as to change their temperature in order to effectuate contractions or expansion of the involved components and hence change dimension of the gap. A passive system, on the other hand, relies on the surrounding environment to effectuate the gap closure. Examples of an active clearance control can be had by referring to U.S. Pat. No. 4,069,662, granted to Redinger et al on Jan. 24, 1978 and assigned to the assignee common to this patent application. Examples of a passive clearance control system can be had by referring to U.S. Pat. Nos. 3,575,523, granted to F. J. Gross on Apr. 20, 1971; 4,534,701 granted to G. Wisser on Aug. 13, 1985 and 4,863,348 granted to W. P. Weinhold on Sep. 5, 1989.
One method of reducing the leakage of air across the tip of the turbine is to discreetly inject the discharge air from the turbine blades internal cooling passages at judicious locations at the tip of the blade adjacent the pressure side of the blade. This serves to create a buffer zone and forms a curtain of air to effectively minimize the leakage occurring across the tip of the blade from the pressure side to the suction side.